1. Field of the Invention
The present invention relates generally to refractory solid-state material heat pipes, and particularly to heat pipes conformally made into heat shields, panel radiators for space power heat radiation, or other purposes in which pipes and surface shields are capable of operating at low and high temperatures in either oxidizing, inert, vacuum, or meteor-filled environments.
2. Description of Related Art
The heat pipe is a thermal device for the efficient transport of thermal energy. It is a closed structure containing a working fluid that transports thermal energy from one part, called the evaporator, where heat is supplied to the device, to another part, called the condenser, where heat is extracted from the device. This energy transport is accomplished by means of liquid vaporization in the evaporator, vapor flow in the core region, vapor condensation in the condenser, and condensate return to the evaporator by capillary action in the wick. The basic idea of the heat pipe was first suggested in 1942, but it was not until 1963 that an actual device based on this idea was built.
Since pressure variations inside the vapor core are normally small, the heat pipe is usually very nearly isothermal and close to the saturated vapor temperature corresponding to the vapor pressure. The capability of transporting large amounts of thermal energy between two terminals (evaporator and condenser) with a small temperature difference is equivalent to having an extra-high thermal conductivity according to Fourier's law. It is like a superconductor in the thermal sense. In addition to its superior heat transfer characteristics, the heat pipe is structurally simple, relatively inexpensive, insensitive to a gravitational field, and silent and reliable in its operation. It can be made into different shapes and can operate at temperatures from the cryogenic regions up to structurally limited high-temperature levels by using various working fluids, ranging from cryogens to liquid metals.
The heat pipe in many aspects is similar to the thermal siphon which has been used for many years. If no wick is used for capillary pumping and the condensate is returned to the evaporator by gravity, the heat pipe becomes indeed a two-phase closed thermal siphon, which is sometimes called the gravity-assisted wickless heat pipe.
Known liquid heat pipes have excellent heat transfer abilities by virtue of the liquid and vapor transport along the heat pipe--hot to cold end. These liquid heat pipes are non-suitable for space applications, resulting from the lack of gravity, meteorite environment, hot and cold extremes, and general internal structural fragility. A puncture or fracture of the casing will yield a complete failure of the unit by leakage and loss from freezing, and it is desired to obviate these problems with a more robust design approach.
Heat shields or heat sinks, and the like, are currently undergoing rapid development utilizing materials including carbon-carbon composites, silicon-based materials, or similar sandwich composites for utilization in ultra-high temperature or ultra-cold temperature environments. However, none of the known materials, structures, or articles of manufacture, designed to protect surfaces operating at extreme temperatures, are capable of functioning without some form of internal cooling fluid generally flowing through cooling channels or tubes within the composite or article.